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CONTRIBUTION FROM THE DEPARTMENT OF BOTANY 
OF COLUMBIA UNIVERSITY 



THE RELATION OF THE HEALTH OF THE 

HOST AND OTHER FACTORS TO INFECTION 

OF APIUM GRAVEOLENS BY 

SEPTORIA APII 



H. E. THOMAS 



SUBMITTED IX PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE 

DEGREE OF DOCTOR OF PHILOSOPHY IN THE FACULTY OF PURE 

SCIENCE, COLUMBIA UNIVERSITY 



NEW YORK 

1921 



Reprinted, without change of paging from the Bulletin of the Tokrey Botanical 
Club 48: 1-29. January 24, 1921. 



CONTRIBUTION FROM THE DEPARTMENT OF BOTANY 
OF COLUMBIA UNIVERSITY 



THE RELATION OF THE HEALTH OF THE 

HOST AND OTHER FACTORS TO INFECTION 

OF APIUM GRAVEOLENS BY 

SEPTORIA API! 



H. E. THOMAS 



SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE 

DEGREE OF DOCTOR OF PHILOSOPHY IN THE FACULTY OF PURE 

SCIENCE, COLUMBIA UNIVERSITY 



NEW YORK 
1921 



Reprinted, without change of paging from the Bulletin of the Torrey Botanical 
Club 48: 1-29. January 24, 1921. 



I 



Gift 

^Diversity 
MM is is$i 



[From the Bulletin op the Torrky Botanical Club, 48: 1-29. 20 Ja 1920.] 



V5 



The relation of the health of the host and other factors to infection 
of Apium graveolens by Septoria Apii 

H. E. Thomas 

Students of immunity and susceptibility have been slow to 
recognize any fundamental distinctions in the relations of host 
and parasite in the great group of organisms which cause disease 
in plants and' animals and yet the concepts of saprophyte, semi- 
saprophyte, and obligate parasite have been current at least since 
the time of DeBary. Under the influence perhaps chiefly of 
Ehrlich's side chain theory of immunity, degrees of resistance have 
been regarded on the one hand as inversely parallel to the virulence 
of the attacking organism and on the other hand as directly 
parallel to the vigor of the host. In plant pathology this view has 
been particularly prominent in the literature of the facultative 
parasites. With the development of the science of immunity, 
the animal pathologist has gone so far as to regard the interactions 
of host and parasite as specific in each case. It is becoming 
increasingly apparent that the specificity in the relation of plant 
pathogens with their hosts must be reckoned with. The sapro- 
phytic fungus may be able to live on dead tissue from a wide range 
of plants, sometimes showing little preference for any one of them. 
The semisaprophyte may or may not be more limited in its food 
range on dead material and attacks from one to a considerable 
number of living plants with varying degrees of virulence and 
with variable results to the hosts. The obligate parasite is usually 
still more restricted in its host range and is much more closely 

1 



2 Thomas: Infection of Apium graveolens 

adapted to the living host, having completely lost the ability to 
grow on dead tissue even that of its most common host. In the 
more highly specialized forms, the relation may become specific 
to such a degree that a comparatively slight change in either host 
or fungus will completely change the virulence of the parasite or 
the effect on the host. It is to be expected, after the long period of 
association necessary for the close adaptation of fungus to host, 
that both would be more or less similarly influenced by their 
environmental conditions. I shall present data to show that the 
infection of Apium graveolens by Septoria Apii is favored by con- 
ditions which accelerate the growth of the host. 

HlSTOCICAL INTRODUCTION 

The late blight of celery was first reported in the United 
States in 1891 by Chester (4) and Humphrey (15). It had been 
described the previous year from Italy by Briosi and Cavara (3), 
probably the earliest record of the disease. Prillieux and Dela- 
croix (29) reported it in France in 1894, and Sorauer (35) recorded 
its occurrence in Germany in severe form in 1896. It came to 
the notice of Salmon (34) in England about 1906. By 1897 the 
disease in America had spread to California and was reported by 
Rogers (31) as a serious pest in that state. An interesting sug- 
gestion as to the possible source of origin of the disease on culti- 
vated celery comes from Pethybridge (26). This author found 
on wild celery a fungus which appeared to be identical with the 
form on the cultivated crop. He was able to transfer it to culti- 
vated celery and produce typical infection. The wild form was 
growing in comparatively complete isolation and Pethybridge 
believes it very unlikely that the fungus could have spread from 
cultivated plants to this host. Hence he concludes that the wild 
form is probably the source of the pest. Dorogin (9) in Russia 
reports a new species of Septoria on celery, S.'Apii graveolentis, in 
addition to the common species. The former is said to be more 
destructive than the latter. During the early and middle parts 
of the growing season no severe infection of celery is usually 
noted, although Salmon (34) found that serious damage was done 
throughout the growing season. Further study of the relation of 
this disease to temperature and other seasonal conditions is 
needed. 



Thomas: Infection of Apium graveolens 3 

Duggar and Bailey (10), Clinton (5 p. 267), and Link and Gard- 
ner (19) have observed that both celery and celeriac are attacked 
by Septoria in storage. The first authors noted that over 50 per 
cent of the stored crop was made unsalable by this fungus in 
one instance. 

No extensive observations seem to have been made on the 
degree of susceptibility of the commercial varieties of celery to 
the late blight. Salmon (4) tested several varieties and arranged 
them in the following series, the first being most susceptible: 
Solid White, Clark's Early Market, Superb Pink, Giant Red, 
Standard Bearer, celeriac. Howitt (4) noted that Golden Self 
Blanching was particularly susceptible. White (42) concluded 
in general that Golden Self Blanching was very susceptible to 
disease while Henderson's Easy Blanching was comparatively free 
from disease. On wild celery Pethybridge (26) found fewer and 
smaller spots produced by Septoria, which seemed never to kill 
the leaf entirely. He states that the fungus occurs on parsley 
though rarely in the British Isles. Cooke (7) and others have 
considered the forms on celery and parsley identical, although I 
have found no record of the transfer of the Septoria from one of 
these hosts to the others. 

Some interesting observations were made by Kinney (16) on 
the relation of cultural practice to blights of celery. He found 
that the amount of disease was reduced by mulching with seaweed, 
soil, coarse manure or even blighted leaves. Plants grown in the 
shade of trees seemed to be less subject to "blight." Unfortu- 
nately the author did not distinguish between Septoria and Corco- 
spora blight. Zobel (43) believes that reduction of the amount of 
manure in the trenches and top dressing the soil with kainit 
greatly reduced the amount of Septoria in England. 

Attempts to avoid or control the late blight, while not uni- 
formly successful, are in practical agreement in the essentials. 
Rogers (31), Salmon (34), Howitt (14), Coons and Levin (8), and 
Krout (18) obtained practical control of the disease with Bordeaux 
mixture. 

Materials and methods 

I have attacked the problem of the relation of health, age and 
other conditions to susceptibility by means of inoculation experi- 



4 Thomas: Infection of Apium graveolens 

ments with celery in the greenhouse, where the health of the host 
was altered by various experimentally controlled conditions. 
Each set of plants was accompanied by control plants and no 
attempt has been made to compare in any detail plants inoculated 
at different dates or plants which for any reason cannot be referred 
to the same control plants. Some experiments with hosts other 
than celery were made to show the host range of the Septoria 
and will be taken up in detail later in the paper. The plants 
were grown in pots usually in garden oil which was screened and 
mixed to obtain uniformity. Inoculations were made by atomiz- 
ing with a suspension of spores taken from infected leaves. The 
plants of a series were placed in a group, alternating test and con- 
trol plants, and were atomized from above and from the sides to 
obtain maximum dosage. The spore suspension was diluted in 
one case I to 10 and in another I to 20 without any marked de- 
crease in the amount of infection produced. The counts in these 
cases were not high, however. The inoculation chamber was a 
rectangular box constructed from window sashes and lined with 
burlap which was saturated with water at the time of inoculation. 
The plants were usually kept in the inoculation chamber about 
forty-eight hours. Since Septoria spreads only slightly in the 
dry atmosphere of the greenhouse and since celery is little 
affected by other pests under these conditions, this is a particu- 
larly convenient form for study. The difficulty of accurately 
measuring the amount of infection was obvious here as in all 
infection experiments. However, when only plants of equal age 
and approximately equal size are compared it seems accurate to 
count the total number of spots per leaf or per plant. This 
method is not satisfactory in older plants in variety tests since 
there is considerable difference in the size of mature plants of 
different varieties. The methods employed for the computation 
of leaf area are too unwieldy for use with plants in any considerable 
numbers. 

Name of the parasite 
The specific name of the Septoria under consideration is in- 
volved in one of the more or less hopeless name tangles which 
serve so frequently to confuse the minds of botanical workers. 
Chester (4) in making one of the earliest reports (1891) on the 



Thomas: Infection of Apium graveolens 5 

fungus in America expresses uncertainty as to the identity of the 
species but includes a description of the fungus and states that if 
it is a new species it "might be named Septoria Apii." Rostrup 
(33) in Denmark (1893) published the same name apparently 
independently. Briosi and Cavara (3) published their variety 
of Septoria Petroselini in 1890 (appeared 1891). Klebahn (17) 
reviewed the situation at some length, examined the exsiccati and 
decided in favor of the name S. Apii Rostr. Quanjer and Slagter 
in Holland (30) and Coons (8) in this country have treated the 
fungus under the same name. It will be noted later that the 
fungus I have studied, so far as it has been tested, does not produce 
infection on parsley either in the greenhouse or in the field. It 
has already been pointed out that no report of cross inoculations 
has been found in the literature, although a number of authors 
have taken it for granted that the form on parsley and the one 
on celery are identical. Therefore, whether or not it be granted 
that the failure to cross infect between these closely related hosts 
be considered ground for making a specific distinction, I shall 
continue to use the name S. Apii Rostr., especially since it is 
already quite widely distributed in the literature. The question 
of nomenclature may well be left till our knowledge of the life 
history of the fungus is completed by the discovery of an ascigerous 
stage. 

Character of the spot 
The spots on the celery leaves when mature are rounded, 
brownish, and usually quite distinct in outline. In severe cases, 
as the leaves become older and the spots more numerous, the 
tissue between the spots breaks down and the entire leaf may 
wither. Pycnidia may, however, appear while the spot is still 
quite or entirely green. In this respect S. Apii is widely removed 
from those species in which the pycnidia ripen after the develop- 
ment of a well-marked discolored area. The mycelium of the 
fungus is at first intercellular and may spread at least from one 
to two millimeters through the tissue before the cells of the host 
break down. This stage is reached after from ten to fifteen days, 
depending on the temperature, the condition of the host, and 
perhaps other factors. The collapse of the mesophyll and palisade 
cells is quite complete and leaves little more than the epidermal 



6 Thomas: Infection of Apium graveolens 

layers with fragments of leaf cells mingled with the mycelium of 
the fungus making up the spot. Pycnidia begin to form before 
the tissue breaks down, in fact it is not uncommon, as has been 
noted, to see, with the hand lens, mature pycnidia with ostioleson 
tissue which is still green and in which no shrinkage can be detected. 
The pycnidium originates usually if not always in an intercellular 
space, frequently in the substomatal cavity and as it increases in 
size the adjacent cells are broken down and successively become 
replaced by the heavy thick- walled hyphal elements. Occa- 
sionally a portion of a cell may remain intact until its lumen is 
tightly packed with the mycelium. The origin of the pycnidium 
is not necessarily always subepidermal but may be at any point 
in the mesophyll or palisade tissue. On the petiole, however, the 
pycnidia seem to be restricted to the outer layers, none having 
been observed deeper than the third or fourth cell layer. When 
the leaf tissue collapses the pycnidia become more conspicuous, 
projecting above the general level of the spot. From the sharp- 
ness of the margin, which appears macroscopically between the 
spot and the surrounding tissue, it might be concluded that the 
margin of the mycelial growth is coincident or nearly so with the 
margin of the spot. In sections, however, the mycelium is found 
at a distance of several cell diameters in advance of the breaking 
down of the tissue. It has been noted further that pycnidia are 
visible with low magnification in the green margin surrounding 
the spot. As the spot ages, there is a reduction in the green color 
of the tissue immediately surrounding it. Strangely enough, 
however, when the leaf yellows from age this partially yellowed 
region retains its color longer than any other portion of the leaf. 
Whether or not the green is intensified in this area as it diminishes 
elsewhere, I am not able to say from my observations. However, 
it is plain that in some way the fungus has caused the prolongation 
of the life of the cells in this region beyond that of the cells of the 
remainder of the leaf. This is further evidenced by the fact that 
on petioles which are wilted gradually, the loss of both chlorophyll 
and water is inhibited in the vicinity of infected spots, especially 
the smaller spots (which do not lose any considerable amount of 
water through the dead tissue). This condition was especially 
marked in a plant in the field which had its lower roots cut by 
mice and was wilting slowly. 



Thomas: Infection of Apium graveolens 7 

On the seed the fungus does not produce a definite spot. 
The pycnidium is found imbedded in the pericarp with only a 
small spreading of the mycelium into adjacent tissue. In the 
sections studied, the mycelium did not penetrate to the embryo. 
That this may occur, however, seems reasonably to be expected 
and it would probably result in most cases in inhibiting germina- 
tion of the seed. 

Cultural characters of the fungus 
The fungus grows readily although slowly on a variety of 
media in pure culture. On starch,* beef peptone, and celery 
decoction agars the fungus produced somewhat greater radial 
growth at comparatively low temperatures (i3°-i9° C.) than at a 
temperature ranging from 22 to 27 C. On starch agar, which 
was the most satisfactory medium tested, a colony 15-18 mm. in 
diameter was produced in four weeks with mature pycnidia and 
considerable superficial fine white mycelium. As the culture 
ages the mycelium becomes coarser and darker until finally the 
surface of the medium is covered with a dense black weft. On 
the agar prepared from a decoction of celery leaves, the growth 
is similar to that described above but less vigorous. In marked 
contrast is the colony produced on ordinary beef peptone agar. 
The mycelium is dark and coarse from the beginning, radial 
growth is small and the result is an irregularly pulvinate colony 
very densely compacted. A somewhat similar growth was ob- 
tained on steamed coconut, bean stems, and petioles of celery 
and beet. When celery leaves were mixed with garden soil and 
steamed, no growth of the fungus could be obtained. Clean 
white sand was substituted for the soil and a good vigorous growth 
followed even spreading into the sand adjacent to the celery tissue. 

Specialization of Septoria Apii as to hosts 

It has been accepted by a number of pathologists in Europe, 
America and elsewhere that the Septoria of celery is transferable 
to parsley and vice versa, although I have found no record of 
inoculations to settle this question definitely. I have attempted 
to determne to what extent the celery fungus has become special- 

* Czapek's formula, with 10 gm. corn starch substituted for the sugar. 



Thomas: Infection of Apium graveolens 



ized in its choice of a host by inoculating plants from the following 
groups : (a) plants of the family Umbelliferae, including varieties 
of celery; (6) miscellaneous plants, nearly all of which are known 
to be hosts of Septoria. The varieties of celery were tested both 
in the greenhouse and in the field. Table I shows the relative 

TABLE I 

Relative susceptibility of varieties of celery under greenhouse 
conditions* 



Variety 



Average number of spots per plant 



No. of 
plants 


First inocu- 
lation 


Second inocu- 
lation 


12 
10 


15-7 
8.4 


170.5 
167.O 


12 


15.2 


124.I 


12 


4.0 


103.0 


12 
12 


2.4 
6.1 


51-4 
43-7 


II 


3-2 


23-0 



White Plume 

New Rose 

Golden Self Blanching 

Giant Pascal 

Winter Queen 

Golden Half Dwarf. . . 
Celeriac 



susceptibility of six of the common varieties of celery and of 
celeriac under greenhouse conditions. The plants grown in the 
greenhouse were six to eight inches high at the time of the first 
inoculation (June 26 and June 30), and were quite uniform in size 
(except the variety, Giant Pascal, which was somewhat larger). 
The plants were not in a vigorous growing condition judging from 
the yellowing of outer leaves and the slow rate of growth. It will 
be noted that the first inoculation produced comparatively low 
counts. The plants were inoculated again without repotting on 
August 27 and August 29 when conditions were more favorable 
for infection. The data as far as they go suggest that there is 
some consistent difference in varietal susceptibility, although no 
variety shows any pronounced resistance. The white varieties, 
Golden Self Blanching and White Plume, show especial suscep- 
tibility. In the field the variety test included the varieties 
Boston Market and Henderson's Easy Blanching, in addition to 
the varieties used in the greenhouse. The estimation of damage 
done by the fungus was much less simple in the field than in the 

* A* number of these plants were found to be infested by nematodes. However, 
among the lowest counts resulting from the infestation were those of three plants 
of the variety, White Plume, which stands highest in the total counts. Hence it 
does not seem that the presence of the nematodes materially affects the position of 
the varieties as presented here. 



Thomas: Infection of Apium graveolens 9 

greenhouse. The season was favorable for vigorous development 
of the fungus and as a result the spots soon ran together and 
caused the collapse of the entire leaf. Consequently the method 
of counting spots could not be used. The total weight of celery 
produced would be inaccurate as an index of the severity of the 
attack, since the varieties differ normally in the weight of the 
mature plant. However, certain general conclusions can be 
drawn from the gross appearance of the plants at the end of the 
season. The plants were inoculated by atomizing a single plant 
of each variety on August 5. On October 29, at the time of digging, 
the variety Golden Self Blanching had been so severely damaged 
that only a few living leaves remained. During the latter part of 
the season while the plants were blanching, a soft rot was asso- 
ciated with the late blight on all of the varieties. This was 
especially severe on the Golden Self Blanching and seemed to 
follow in areas of dead tissue killed by the Septoria, especially on 
old leaves. The unusually wet period at that time would have 
favored the development of the various saprophytes which are 
present under such conditions. Of the varities other than Golden 
Self Blanching, there was no easily recognizable difference in 
susceptibility. Easy Blanching (Henderson's) seemed to with- 
stand the blight and subsequent rot slightly better than the other 
varieties. White Plume, a self blanching form, was not notice- 
ably poorer than the green varieties. An accurate method of 
estimating the amount of infection would probably have shown 
differences which could not be noted with certainty from the 
general appearance. 

Infection tests on various Umbelliferae and a considerable 
number of miscellaneous plants have shown that the species of 
Septoria under consideration here is very limited in its host range, 
if not entirely restricted to the single species, Apium graveolens, 
and its variety rapaceum. Parsley {Petroselinum sativum) has 
been inoculated repeatedly in the greenhouse under controlled 
conditions but no sign of infection has been produced. The 
tests included the plain leaved parsley (two varieties), the curly- 
leaved type and the Hamburg or turnip-rooted parsley. The 
plain parsley was grown in the field in a row adjacent to heavily 
infected celery but infection was never found on any of the 



10 Thomas: Infection of Apium graveolens 

plants. Whether there are other strains of the fungus which in- 
fect both celery and parsley is a question of interest, both theoret- 
ically and practically, and should receive further attention. The 
various other hosts tested were grown in pots in the greenhouse 
and inoculated with celery plants in every case to check on the 
conditions for infection. The following plants were tested: 

Umbelliferae 
Anethum graveolens L., dill 

Anthriscus cerefolium (L) Hoffm., chervil (beaked parsley) 
Carum Carvi L., caraway 
Coriandrum sativum L., coriander 
Cryptotaenia canadensis (L.) DC, hone wort 
Daucus Carota L., carrot (both wild and cultivated) 
Foeniculum officinale All., fennel 
Osmorhiza sp., sweet cicely 
Pastinaca sativa L., parsnip 

Petroselinum sativum Hoffm., parsley (plain leaf, curly leaf, 
and Hamburg varieties) 
Silaus Besseri 

Miscellaneous 

Antirrhinum majus L., snapdragon 

Beta vulgaris L., beet (sugar and garden varieties) 

Lactuca sativa L., lettuce 

Lobelia sp. 

Ly coper sicum esculentum Mill., tomato 

Nicoliana Tabacum L., tobacco 

Pisum sativum L., pea 

None of the plants listed here developed any sign of infection. 
From these data it must be concluded that the Septoria of celery 
has reached a comparatively high degree of specialization as to 
its hosts. These results agree essentially with those of Beach (i), 
working with a considerable number of other species of the genus 
Septoria. 

Effect of fertilizers upon infection 
Realizing that the terms health, vigor, and vitality are vague 
and difficult of definition in plants as in animals, I have attempted 



Thomas: Infection of Apium graveolens 



11 



to modify these conditions in celery plants by various methods of 
feeding and handling to determine the influence of such treatment 
upon the interaction of host and parasite. The direct effect upon 
the plants has been visible in some cases in the increase or decrease 
in growth, the putting out of new leaves or the dropping of old 
leaves and in the turgidity of the tissues. In other instances the 
reaction to the treatment was not so directly evidenced. The 
difficulty in obtaining properly controlled results is obvious, but 
I have made a number of experiments to test the amount and 
character of the infection produced by inoculating plants in 
different conditions of health more or less artificially induced. 

One of the first striking results noted was that which was 
produced by treating pot bound plants with sodium nitrate in 
solution. Five plants in four-inch pots of garden soil received 
each i gram of sodium nitrate in ioo c.c. of water. In this and 
in the succeeding experiments, the control plants received an 
amount of water equivalent to that used in the solution with the 
nutrient. The plants were inoculated at the time the nitrate 
solution was added. 

Table II 
Increase in infection produced by treating pot bound plants with sodium 

nitrate solution 



Plant No. 


1 


2 


3 4 


5 


Average No. 
Average spots per 
No. leaves i ] ea f 


Sodium nitrate ' 34 8 

Control 1 234 


185 
97 


177 
II 


189 
43 


238 
38 


10.6 24.3 
8.0 10.6 



Table II shows the very marked increase in infection obtained 
upon the plants which received the fertilizer. This difference is 
unusually marked due to the fact that the plants were badly pot 
bound and growth had been markedly checked. 

With the garden soil used, the addition of calcium sulfate 
in the dry form, as it has been used in agricultural practice, pro- 
duced a small decrease in infection. This series was prepared by 
mixing about five grams of calcium sulfate with the soil of each 
pot at the time of repotting. Twenty-four days later these plants 
were inoculated with controls. After the records were taken 
these plants were kept upon the greenhouse bench in compara- 



12 



Thomas: Infection of Apium graveolens 



tively dry atmosphere until the infection had largely been thrown 
off. They were again inoculated July 10, nine weeks after the 
first inoculation. The results of both inoculations are shown in 
Table III. The lower counts on the second inoculation may be 



TABLE III 
Decrease in infection upon plants treated with calcium sulfate 





First inoculation 


Second inoculation 


Spots 
per plant, 
first inocu- 
lation 


Spots 
per plant, 




I 


2 


3 


4 


5 


6 


1 


2 


3 


4 


5 


6 


second 
inoculation 


CaS0 4 

Controls 


387 
623 


281 
368 


266 

495 


301 
350 


271 505 
34^ 382 


87 
94 


28 
113 


56 14 
172 99 


80 
80 


79 
36 


335-1 
426.6 


57-3 

99.0 



explained partly by the fact that the plants had by this time be- 
come pot bound and partly by the influence of seasonal conditions, 
for during the warm weather of midsummer, no high infection 
counts were obtained on any plants in the greenhouse regardless 
of their condition. It will be seen that there is a consistent de- 
crease in the counts on the treated plants from both inoculations. 
The plants were not appreciably altered in appearance by this 
application. 

TABLE IV 
Effect upon infection of feeding with various fertilizers and in one case 
top dressing with lime 



Plant No. 



Controls 

KH2P04 

Ca(N0 3 ) 2 andKNOa 

Complete nutrient 
solution 

Sheep manure 

Hvdrated lime 
(CaCOa) 



246 
506 
412 

559 

494 

103 



607 
412 
429 

662 

772 

259 



168 
778 
723 

1.237 
461 

5i8 



143 
318 
286 

276 
576 

394 



614 
385 
779 

1,009 

522 

430 



799 
925 
982 

1,242 



460 

1,117 

645 

543 



I-I39 
592 

748 

403 



Average 

No." 

leaves 



5891196 
I44;362 
260 376 



247 



429 



6.0 
6-5 
6-3 

6.9 

5-0 

5-6 



Average 

No. 

spots 

per leaf 



82.6 

85-2 
89-5 

95-7 
113-0 

60.8 



With these results in mind further tests were planned to include 
complete fertilizers as well as their components. A nutrient 
solution was prepared according to a Pfeffer formula (27): 4 
gm. calcium nitrate, 1 gm. potassium nitrate, 1 gm. magnesium 
sulfate, 1 gm. potassium acid phosphate, 0.5 gm potassium chlo- 
ride, trace of ferric chloride, were dissolved in 3 liters of water. 



Thomas: Infection of Apium graveolens 13 

Ten plants received each ioo c.c. of this solution. Two other 
sets of ten plants were treated respectively with nitrates and 
phosphates equivalent to the amounts fed to the first set in the 
complete nutrient solution. Five plants received 5 gm. each of 
hydrated lime on the surface of the soil and the soil of five others 
was top dressed with sheep manure. All of these plants were 
inoculated together with controls immediately after the addition 
of the fertilizers. The infection counts from them are shown in 
Table IV. All of the plants seemed vigorous in their gross 
appearance except those treated with lime. In this case the 
roots in the upper inch or so of the soil were discolored and some 
appeared to be killed outright. The leaves appeared somewhat 
less turgid and vigorous than those of the other series. 

The small margin of difference between the controls and 
treated plants, especially in the case of the phosphate, makes the 
results appear doubtful. It must be borne in mind, however, 
that the concentration of the solutions was that recommended 
for water cultures and did not result in marked increase of growth 
in the treated series. When it is considered that each plant in 
the nitrates series receives only 0.166 gm. of the salt, it will be 
seen that striking results cannot be expected except with the lime 
and manure whch were applied in considerable quantity. How- 
ever, the results are in accord with those of the other experiments 
reported here. Whether or not the increase in new growth under 
field conditions would enable fertilized plants to increase the total 
yield in spite of increased infection is of course not shown by 
these experiments. 

In watching the plants from day to day it seemed that not 
only was there an increase in the number of spots on plants 
treated with fertilizer but there was also a tendency toward the 
formation of larger spots and more rapid breaking down of the 
tissue between the spots. In the field, where conditions were 
more favorable for the growth of the host than could be supplied 
in pots in the greenhouse, a count of our nineteen hundred spots 
was obtained from a single leaf (Easy Blanching) and, as will be 
pointed out later, the older leaves regularly withered entirely 
from the coalescing of the spots. It is true that the field conditions 
of the season in question were also more favorable for the develop- 



14 Thomas: Infection of Apium graveolens 

ment of the fungus than the greenhouse conditions. However, 
the fact that a single leaf in the field bears more infections than 
any count obtained on an entire plant in the greenhouse is note- 
worthy. To obtain some statistical evidence of the relation of 
the fertilizer treatments to the size of the spot, two sets of five 
plants each in three-inch pots were treated respectively with 2 
grams of hydrated lime per plant as top dressing, and 1 gram of 
sodium nitrate per plant in solution. The effect upon both the 
number and extent of infection areas is shown in Table V. 

The nitrate plants average 284 spots per plant as opposed to 
120 spots per plant on the limed plants and, what is perhaps even 
more significant, the difference in the size of the spots is propor- 
tional, the spots of the nitrate plants averaging 2.64 mm. in 
diameter while those of the limed plants average only 1.06 mm. 
The ratios are as 1: 2.36 and 1: 2.54, respectively. These data 
indicate that the degree of susceptibility is dependent upon the 
interchanges between the host cells and fungus hyphae rather 
than upon the ability or lack of ability of the fungus to penetrate 
the host. 

In a further attempt to produce varying conditions of health 
in the experimental plants, sets of five plants each were watered 
with 50 c.c. of each of the following solutions: 2 per cent sodium 
chloride, I per cent magnesium chloride, 1 percent barium chloride, 
0.1 per cent ferric chloride, 0.1 per cent zinc chloride. These 
were inoculated with controls as in the preceding experiments. 
The results are not sufficiently uniform to be considered significant. 
The health and growth of the plants were not perceptibly altered 
and the amount of infection was fluctuating. The plants treated 
with magnesium, iron and zinc were somewhat lower in total 
counts than the controls, while the barium series gave the highest 
counts of all. It is to be remembered that our lack of knowedge 
in regard to the behavior of these substances in relation to the soil 
and to the selective absorption phenomena exhibited by the roots 
of plants would make any but the most striking results extremely 
difficult of interpretation. 

Bearing upon the question of the relation of fertilizers to 
infection is the following experiment, which was begun with other 
matters in view. Five five-inch pots were filled about one-third 



Thomas: Infection of Apium graveolens 



15 



TABLE V 
Comparison of the number and si-e of spots on plants treated with sodium 
nitrate and hydrated lime. notes taken twenty days after inoculation 



Treat- 


Plant 

No. 


Leaf 
No. 


Diameter of spots in millimeters 


Aver. 


No. 
spots 
per 


No. 
leaves 


ment 


1 | 




per 








i | 2 1 3 | 4 


5 


6 ' 7 \ 8 ! g 1 10 




plant 


plant 






I 


0-5 


0.5 0.5 


0.5 


0-5 


0.5 1.0 


0.5 1.0 


0-5 


0.60 








I 


2 


1.0 


0-5 0.5 


0.5 


0.5 


0.5 1.0 


0-5: 0.5 


o-5 


0.60 


113 


5 






3 


0-5 


0.5 1.0 


0.5 
3-0 


0-5 
3-5 


0.5 o.s 


1.0 0.5 


0.5 


0.60 










i 


5-0 


3-0 5-o 


3-0 


3-0 


5.01 6.0 


4.0 


3-95 










2 


2-5 


2.0 1.0 


1.0 


2.0 


1.0 


2.0 


1.5 1.0 


1.0 


1.50 








2 


3 


3-o 


2.0 1.0 


0-5 


1.0 


0.5 


i-5 


i.o 1 0.5 


o-5 


I-I5 


159 


6 


a 




4 


0-5 


1.5 1.0 o-s 


1.0 


0.5 1.0 


1.0 0.5 


1.0 


O.85 








5 


0-5 


1.0 0.5 0.5 


0-5 


0.5 0.5 


1.0 0.5 




0.6l 






X) 
























-u 




i 


i-5 


1.0 1.5 2.0 


4.0 


1.0 0.5 


1.0 3.0 


1.0 


I.65 






u 


3 


2 


1.0 


1.0 1.0 1.0 


0-5 


0.5 


1.0 


0.5 0.5 


2.0 


0.90 


159 


6 


>> 




3 


1.0 


0.5 1.0 1.0 


0.5 


0.5 


1.0 


0.5 0.5 I.O 


0-75 










i 


0-5 


1.0 0.5 


1.0 


0.5 


1.0 


i-5 


I.O I.O 


0-5 


O.85 








4 


2 


1.0 


i-5o.5 


0-5 


1.0 


0.5 


1.0 


0.5 I.O 


1.0 


O.85 


135 


5 






3 


0-5 


1.0 0.5 1.0 


1.0 


0-5 


1.0 


i-5,o.5 


1.0 


O.85 










i 


i.o 1.5 1.0 


0-5 


1.0 


0.5 


0-5 


1.0 0.5 


1.0 


O.85 








5 


2 


0.5I0.5 0.5 












0.50 


36 


6 






3 


1.0 


0.5 0.5 


0-5 


0-5 


0.5 


0.5 


o.5 ! 0.5 1.0 


0.60 










I 


3-o 


2-5 


5-o 


4.0 


6.0 


4.0 


3.0 4.0 4.0 


7.0 


4-25 










2 


5-0 


5-0 


5-o 


5-o 


5-0 


6.0 


5.0 6.0 


7.0 


6.0 


5-40 


459 


7 






3 


4.0 


3-0 


3-o 


5-o 


1.0 


3-0 


3-o 


i-5 


3-0 


3-0 


2-95 






4 


2.0 


2.0 


1.0 


1.0 


1.0 


i-5 


2.0 


i-5 


i-5 


2.0 


1-55 










i 


7.0 


6.0; 5.0 


6.0 


6.0 


6.0 


6.0 


7.0 


5-0 


5-o 


5-90 










2 


3-0 


4.0 


5-o 


4.0 


5-0 


4.0 


2.0 


3-0 


3-0 


3-o 


3-6o 








2 


3 


3-o 


5-o 


7.0 


4.0 


5-0 


2-5 


4.0 


i-5 


2-5 


5-o 


3-95 


314 


8 






4 


2.0 


4.0 


3-o 


1.0 


3-0 


4.0 


3-0 


3-0 


4.0 


2-5 


2-95 






CI 




5 


2.0 


1.0 


2.0 


i-5 


i-5 


2.0 


2-5 


1.0 2.0 


i-5 


1.70 






u 




I 


2.0 3.0 2.0 


3-o 


3-0 


i-5 


1.0 2.0 2.0 


1.0 


2.05 






a 

S 

3 


3 


2 


1-5 1-0 


i-5 


2.0 


1.0 


i-5 


1.0 1.0 1.0 


1.0 


I-I5 


177 


9 




3 


i-5 0.5 


1.0 


1.0 


0.5 


1.0 


1.0 1. 01 3.0 




1. 16 






■a 
o 

w 




i 


5-o 


2.0 1.0 


2-5 


3-5 










2.80 










2 


i-5 


6.0 3.0 


i-5 


4.0 


2.0 


4.0 1.0 


3-o 


1.0 


2.70 








4 


3 

4 


2.0 
3-o 


2.0 
3-5 


3-o 
2.0 


4.0 
2.0 


2-5 
3-o 


3-5 
2.0 


2.0 1.0 

3-o 3-0 


2.0 

i-5 


4.0 
1.0 


2.60 
2.40 


145 


10 






5 


1.0 


0-5 


0.5 


3-o 


2.0 


1.0 


0.5 1.0 


0-5 


1.0 


1. 10 










6 


0.5 


0.5 


1.0 


i-5 


i-5 


1.0 


0.5 0.5 


i-5 


1.0 


1.05 










I 


2-0 3.5,3.5 


3-o 


1.0 


1.0 


0.5 1.0! 0.5 


3-0 


1.90 








5 


2 

3 


3.0 3.0 2.0 
1.0 1.0 2.0 


2.0 
2.0 


4.0 
2.0 


1.0 
2.0 


3.0 i.oj 2.5 
2-5 3-0 3-5 


2-5 
2.0 


2.40 
2.10 


325 


7 






4 


1.0 0.5 0.5 


0.5 


2.0 


0.5 1.0 0.5' 0.5 


I.O 


0.85 







16 



Thomas: Infection of Apium graveolens 



full with heavily diseased leaves and petioles of celery. Five pots 
were similarly filled with healthy green leaves. Celery plants 
were potted in garden soil in the upper part of these pots and 
twenty days later these plants were inoculated. The result was a 
very decided increase in infection upon the plants which received 
the diseased material (Table VI). While there was no perceptible 

TABLE VI 

Infection on plants treated with diseased celery leaves in the soil 



Plant No. 


i 

260 

9 

159 

13 


2 

288 

7 

217 

7 


3 

372 

6 

255 

13 


4 


5 


Average 

No. spots 
per leaf 


Plants with diseased material 
Controls 


Spots per plant 

Leaves per plant . . . 

Spots per plant 

Leaves per plant . . . 


301 
9 

132 
8 


153 

6 
187 

7 


38.7 
19.7 



difference in the appearance of the two series of plants, the most 
reasonable explanation of these results is that the diseased material 
with decay already under way would supply, at the time of inocu- 
ation, a condition similar to that produced by the application of 
manure made in an earlier experiment. The depth at which the 
material was buried and the uniformity in time with which the 
infection appeared preclude the possibility of the action of this 
material as an additional source of inoculum. 

Infection of plants infested by nematodes 
Perhaps the most conclusive data bearing on the relation of 
health to infection were obtained incidentally in the course of 
experiments planned for other purposes. Twelve plants which 
had received various treatments were given two successive 
inoculations, which produced at most only a few spots. These 
were finally thrown out and the roots were found to be heavily 
infested by nematodes. Again nine plants were set aside from a 
variety test as possibly resistant individuals. These were also 
found to be infested. The infection (Septoria) on these plants is 
compared with that on two non-infested plants of the same experi- 
ment in Table VII. So far as these observations extend, nema- 
todes do not develop to any marked degree on any except badly 
pot-bound plants. However, a number of galls may be produced 
on the roots of plants in fairly good growing condition. A number 



Thomas: Infection of Apium graveolens 



17 



TABLE VII 
Reduction in number of spots produced by Septoria on plants infested by 

nematodes 





Infested by nematodes 


Not 
infested 
















g. 


<~ 


<_ 






Variety 




Z V 
C 0> 


s 8 


7=1 


|a 


is 8 

<U O 


-°.G. 




2Q 


& J! 

<v 


a 




'■ZZ- '^Z' 


£o- 


£E 


zi 


!«tf 


v. 
C a 




015 


£p* 2i 














X 


S3 


w 




O 


No. of leaves 


5 

is 


4 
8 


5 
5 


5 
56 


5 
19 


5 
12 


5 
19 


7 
5 


4 
29 


4 4 

503 ^8s 


Spots per plant 













of these cases were noted in connection with the fertilizer exper- 
iments previously described, in which no reduction in vigor of 
the plant or in the amount of infection could be detected. An 
attempt to produce the infestation by inoculation into the soil 
was made with ten young plants in good growing condition but at 
the end of ten weeks, no galls were evident. The history of a 
single plant which has been followed more closely will throw light 
on the role which the nematode plays here. A plant was noted as 
"highly resistant" in the course of an experiment and when reinoc- 
ulated it had lost its susceptibility completely. The roots were 
very heavily infested by nematodes. The plant was placed in a 
larger pot with fresh soil and five weeks later, when considerable 
growth had been made, it was again inoculated with a control. 
At the end of twenty-eight days no spot recognizable as due to 
Septoria could be found on the plant. The control plant bore 219 
spots. Seven weeks after this inoculation the "nematode" plant 
was treated with 300 c.c. of the nutrient solution described above 
with the fertilizer experiments. At this time a few small spots 
could be seen with an occasional pycnidium. The plant was 
finally inoculated six days after the addition of the fertilizer and 
thirty days later a count of 478 spots was obtained. These spots 
were for the most part small and the fructification of the fungus 
was feeble. According to the accounts of the behavior of nema- 
todes in the roots of plants, it seems clear that the foliage is starved 
both by the disruption of the vascular elements and by the with- 
drawal of food materials to produce the galls. Here is undoubtedly 
a clear case of the reduction of infection by a fungus parasite 
running parallel with the reduction in vigor of the host. 



IS 



Thomas: Infection of Apium graveolens 



Infection of etiolated plants 

The effect of etiolation of the host upon infection has been 
tested in various ways. The first series of plants were kept in 
a dark room for nine days just preceding inoculation. Controls 
were kept on the greenhouse benches. The counts of spots per 
plant are shown in Table VIII. This prolonged period in the 



table viii 

Effect upon infection produced by etiolating plants for nine days imme" 
diately preceding inoculation 



Plant No. 


i 


2 


3 


4 


5 


6 


Total No. 
leaves 


Average No. 
spots per leaf 


Etiolated 

Control 


62 
33 


172 
40 


29 
312 


63 
253 


35 
400 


39 
72 


50 
64 


8.0 
16.7 



total absence of light materially changed the plants in a number of 
ways. The most noticeable changes were loss of chlorophyll, 
elongation of the petiole, and reduction in size of the leaflets. The 
reduction in leaf area, however, was plainly not commensurate 
with the difference in amount of infection. [Neither can the 
suspension of photosynthesis be held entirely accountable for the 
less vigorous action of the parasite, in view of the various other 
changes in the host and in the light of the following further experi- 
ments. 

The second series of plants were kept in the dark room for 
three and one half days immediately following their removal from 
the inoculation chamber. The plants were not materially changed 
in appearance and it may be seen at once in Table IX that no 



table IX 
Effect of etiolation for three and one-half days following the removal 





OF PLANTS FROM THE INCUBATION 


CHAMBER 




Plant No. 


1 2 


3 


4 


5 


6 


Total No. 
leaves 


Average 

No. spots 

per leaf 


Etiolated 

Control 


422 163 
623 1 368 


313 

495 


584 
350 


403 

342 


565 
382 


48 

47 


50.9 

54-4 



noteworthy variation was produced in the amount of infection 
as compared with the control plants. It was noted moreover 



Thomas: Infection of Apium graveolens 



19 



that the time* required for the first appearance of the spots was 
practically identical for the etiolated and control plants. 

A third set of plants was kept in the dark room for five days, 
beginning on the thirteenth day after inoculation, at which time 
the spots were just beginning to appear. This dark room was 
provided with a ventilator which caused a continuous circulation 
of air from the greenhouse in which the control plants were kept 
thus providing similar atmospheric conditions for the two sets of 
plants. This experiment was performed during the warm weather 
of July, and as a result the plants kept in the dark room lost several 
of the older leaves. Although the total counts of infections could 
not be obtained it is scarcely to be expected that the number of 
spots would be altered by this treatment. 

TABLE X 

Effect on size of spot of etiolation at different times relative to 

inoculation 

a. Plants kept in dark room nine days preceding inoculation 





Etiolated plants 


Control plants 


Plant No. 


Average size spot 


No. of leaves 


Average size spot No. of leaves 


I 


1.62 


5 


1. 17 


6 


2 


i-5i 


5 


1.16 


9 


3 
4 


1-37 
1-54 


4 
3 


0.91 
0-95 


4 
8 


5 


1-45 


3 


0.94 


4 



b. Plants kept in the dark room from the fifteenth to the eighteenth day after inoculation 



I 


2.08 


3 


1.27 


2 


2 


1. .87 


2 


i.5i 


4 


3 


1.50 


2 


1.07 


S. 


4 


1.80 


1 


1. 81 


3 


5 


1-75 


2 


i-45 


3 


6 


1.78 


3 


1-33 


3 



This seemed to be borne out by counts from the individual 
leaves. The effect upon the size of the spot produced by etiolation 
at this time was readily demonstrable. Table X shows the 
increase in diameter of spots upon plants etiolated both before and 
after inoculation. Except for an occasional leaf (nine altogether) 

* Fromme found (12) that Puccinia coronifera on oats was almost completely 
arrested in its progress during the time in which inoculated plants were kept in the 
dark room. 



20 



Thomas: Infection of Apium graveolens 



ten spots per leaf were measured. Thus for a plant of five leaves 
fifty spots were measured. 

In the case of the first series kept in the dark room before 
inoculation, the most marked increase in the size of the spots 
was on the youngest leaves, which were put out partly or entirely 
while the plants were in the dark room. On these the spots at 
times exceeded in diameter those on the oldest leaves. In the last 
series of plants the increase in size apppeared to be proportionate 
for all the spots. It has been noted that when infected plants 
are placed in the inoculation chamber, for forty-eight hours, a 
zone of at least one half to one millimeter surrounding each spot is 
broken down. That the mycelium of Septoria should advance 
this distance in so short a time does not seem probable. It 
appears rather that the~ weakened tissues of the host plant succumb 
where the fungus is already present. 

Relation of temperature to infection 
It has been recognized since the late blight disease began to be 
studied that it is more severe in the early autumn than during mid- 
summer. I have found this to be true in the greenhouse as well 
as in the field. Several experiments have been performed to test 
the relation of this condition to temperature. Plants were 
inoculated uniformly and divided into two groups which were 
kept through part or all of the incubation period at temperatures 



table XI 
Effect of maintaining inoculated plants at different temperatures 
throughout the incubation period of twenty-one days 



Mean av. temperature 



Number of infections per plant 



10 Average 



21.9 C. 
13-3° C. 



383 272 327 281 189 377 

144 86 120 52 I 3 64 



133 209 

73 I 104 



269 
37 



253-7 

79-4 



differing from 7 to 13 degrees Centigrade (mean average). Five 
sets of from ten to twenty plants each were inoculated. The 
infection develops more rapidly at higher temperatures but later 
counts show usually no striking difference and the individual 
plants vary widely. One set (Table XI) showed a marked 
though not altogether consistent difference in counts. However, 



Thomas: Infection of Apium graveolens 



21 



a second record five days after the first showed an average increase 
of fifteen spots per plant on the plants of the lower temperture. 
One set of plants showed a consideraby higher count on the 
plants of lower temperature after thirty-six days. The results 
are too variable to be in any way conclusive. A factor of probably 
greater importance is the fact that the host plant may be more 
vigorous and make its greatest growth in cool weather with the 
autumn rains. Rolfs (32) states that in Florida celery can be 
grown only as a cool weather crop. Lloyd (20) and Watts (41) 
point out that celery demands cool weather, at least cool nghts, 
for satisfactory growth. However, I have not found any specific 
data to show the optimum temperature for the culture of celery. 

Relation of age tissue to infection 
The absence of any conspicuous sign of infection in the field 
during the early and middle parts of the growing season led the 
earlier workers to believe that young plants were affected only 
slightly or not at all. The more careful observations of later 
workers have already disproved this. I have observed the fungus 
on plants in the various stages from the seed bed to maturity. 

TABLE XII 
Number of spots on leaves of different ages twenty-seven days after 

inoculation 



Leaf No.* 


1 


2 


3 


4 1 5 


6 


7 


8 


9. 


10 


Plant No. I 








18 


209 


254 


75 


17 


50 














35 


193 


6t 


79 










3 








5 


93 


185 


127 


44 


41 






4 











40 


95 


130 


26 


25 


24 


4 


5 








52 


216 


36 


27 


11 








6 








39 


194 


42 


67 


3i 


9 




Totals 








149 045 673 


5ii 


129 


125 24 


4 



In the field a row of young plants was set out on July 17 so that 
they were about half grown when the infection was becoming 
severe on the regular crop (late September) . Counting the number 
of spots per leaf is not practicable under field conditions but it 
was obvious that these young plants were attacked with a 
severity quite sufficient to throw doubt on the idea of a close 

* The leaves are numbered here from the center of root crown outward, i.e., from 
youngest to oldest. 



22 



Thomas: Infection of Apium graveolens 



w 
x 

H H 

^ c/i 

Z W 

° s 

Q O 
W 

S w 

5 S3 



O <: 






w 



c 


Yellowed, 

drying 

Yellowing 

Yellowing 

slightly 

Green 


O i/i 


OX m On « OiiO 
in O t^ m o m Q\ 
m ro ro N « h 


00 
CO 

> 


vO i O i i>0 oo 
m m ro n o\ r»o 

M MM M d 6 d 


IT) 


O o in in o m o 

M mm d m d m ! 


* 


m o o m o o q ; 

M MM M M M M 


1 O iflO m O O m 

N m O m mmmO 


1 O mo o "imio 

" M MM M M O O 


S 


o o o in q o in 

(N MP) O M M O 


o 


o o io o O in in 

M M M M M O O 


ON 


o o in o in o in 

M MO M d M d 


00 


O in o in O m in 

pi mm d m o O 


^ l o lom o m o m 

" 1 ri mm m m m d 


VO 


o in in o O in in 

M m O M M O O 


!? 


m in o o in o o 

N MM (M O M M 


IO iflO in O O m 
»- Tt Om mmmO 


IO OO uiviioifl 

HIT) MM MOmO 


2 


in in o o O in in 

H O M M M d d 


2 


o o o o o in in 

IN MM M M O O 





in o O in in o m 

M MM MOmO 


1 in in o in in o in 

=* | m mm mOmO 


00 


O in o o in in in 

m mm m d d d 




o o O o in in o 

M MM M d d M 


o m o o q m o 

M MM M M ~ M 


■n 


O O O o O in in 

rO MM M M O M 


■* 


o o o q m m o 

CO MM M M O M 


CO 


O o in in o in o 

M MM M M O M 


« 


O in in in in o in 

M MM M O M O 


d 

z 

o 

Cm 

CO 


O in o o o o in 

M MM MMMO 


m ii o i m \o i> 

6 
S5 

"rt 



relation between the age of the 
plant and its susceptibility. 
However, as regards the relative 
susceptibility of leaves of dif- 
ferent ages on the same plant, 
results have been obtained to 
show that there is a definite and 
constant relation between the 
age of the leaf and the number 
of spots produced by inoculation 
(Table XII). 

The leaves which show no 
infection are those which were 
pushed out after the inoculation 
was made. It has been deter- 
mined by tagging the young 
leaves at the time of inoculation 
that a leaf is susceptible as soon 
as it pushes out into view. It 
does not however unfold the 
leaflets so as to be exposed to the 
inoculum on all its surface until 
it has reached a height of from 
two to four inches. This ac- 
counts in part for the smaller 
count on the very young leaves. 
No method has been devised to 
determine whether or not these 
leaves are as susceptible per unit 
area as leaves which have com- 
pletely unfolded. The chief point 
of interest here is that a very 
much larger number of infections 
is established on leaves which 
are still actively growing than on 
leaves which are more mature. 
Another very interesting differ- 
ence in the behavior of leaves 



Thomas: Infection of Apium graveolens 



23 



of different ages in relation to fungus invasion is found in the 
rate of breaking down of the leaf tissue after infection becomes 
established. Except in unusual cases in the greenhouse there 
was little coalescing of spots until some days after the infection 
was evident on all of the leaves. Thus time was allowed for 
measurement of the size of spots even on very old leaves while 
they were still distinct in outline. It was found that for a given 
distance between spots, the older the leaf the more rapidly the 
intervening tissue breaks down. 

Table XIII shows the diameter measured to the nearest half 
millimeter of twenty-five spots on each leaf of a plant nineteen 
days after inoculation. The marked decrease in the size of the 
spots from the oldest to the youngest leaf is at once apparent. 

In Table XIV the averages are shown for a number of other 
plants which were similarly studied. It has been pointed out 
that the margin of the killed area produced by Septoria is not 
identical with the limit of mycelial spread. Whether or not the 
fungus colony exceeds the margin of the spot as far in old leaves 
as in young leaves has not been determined. 

TABLE XIV 
Average size of spots on leaves of the same plant at different ages 



Leaf No. 


i 


2 


3 


4 


5 


6 


7 


8 


9 


No. spots 
measured* 


Age of 
infection 


Plant No. i . 

2. 

3- 




2.18 

1.32 
1.66 


1.22 
0.88 
1.06 


O.94 
O.74 
0.86 


O.72 
O.74 
0.80 


O.64 
0.58 










25.0 
25.O 
25.0 


19 dc 

19 

19 


iys 


4- 
5- 
6. 




1.30 

i-45 
1.80 


1. 10 
0.70 
1.40 


1. 00 
0.70 
1 .25 


0-55 
0.70 
1.20 


0.80 


0.60 








10. 
10. 
10. 


29 
29 
29 


< 


7- 
8. 




1.30 
2.05 


1-35 
2.00 


LIS 
1.20 


0.70 
1. 00 


0.90 
0-95 


0-75 
0-95 


• 77 
.80 


.62 
•75 


■75 


10. 
10. 


29 
29 


' 



A possible relation between the acidity of plant juices and their 
susceptibility to fungus attack has been claimed by Comes (6). 
With this in mind two lots of leaves were collected from a number 
of celery plants, the oldest being included in one lot and the 
youngest in another. These were put through a meat grinder and 
15 gram samples of each were extracted for thirty minutes in 
200 cc. distilled water. The extracts were filtered and 100 cc. of 
the nitrate were titrated against approximately twentieth normal 

* In four leaves less than the number of spots indicated was used. 



24 Thomas: Infection of Apium graveolens 

sodium hydrate solution. A very marked increase in acidity was 
thus shown for the old leaves. The ratio of the readings for 
young and old leaves was 4.6 : 6.3. 

Sorauer (36) states that acidity is higher in etiolated plants 
than in the normal green. It has been pointed out that young 
leaves developed in the dark room bear spots as large as those 
produced on very old leaves. This suggests the possibility of a 
relation between the size of the spot and the acid content of the 
leaf. Obviously, however, no more than a suggestion can be made 
from the data at hand. 

Discussion 

The relations of host and parasite are apparently as different 
for the groups of saprophyte, semi-saprophyte, and obligate 
parasite as are the modes of life of the organisms. It is possible 
to arrange an intergrading series according to the completeness 
of adaptation to the host, from a form such as Botrytis (see 
Blackman and Welsford, 2), which habitually kills the host 
cells before it reaches them and is probably never in intimate 
contact with the living cell, to a form such as the seed fungus of 
Lolium temitlentum (see Freeman, 11), which has reached such 
a high degree of adjustment with the host that it is perpetuated 
entirely in the mycelial form through the seed of the host and 
perhaps never kills any of the host cells. As the adaptation to 
the host becomes more nearly complete, there is an increasing 
tendency to show some of the features of mutualism and sym- 
biosis. Fromme (13) has observed with the angular leaf spot of 
tobacco and Peltier (25) with citrus canker that -infection is 
heavier under conditions which favor the growth of the host. 
Marchal (21) found that infection of lettuce by Bremia Lactucae 
was favored by nitrogen and phosphates and retarded by an excess 
of potash. The experiments described in this paper show 
that Septoria Apii, although it readily assumes the saprophytic 
habit, has become so adapted to its host that the development of 
infection is favored by increased growth in the host, such as is 
produced by feeding the plants with nitrates, with a complete 
nutrient solution, or by top dressing the soil with sheep manure. 
The acceleration is manifested in both the number of infections 



Thomas: Infection of Apium graveolens 25 

established and the size of the spots produced. On the other 
hand top dressing the soil of pots with lime decreases the infection. 
Also the infestation of the roots of celery plants by nematodes 
partially or entirely inhibits the development of the fungus. 
The retention of chlorophyll and water in the tissue adjacent 
to infected spots after these have disappeared from the remainder 
of the leaf is further evidence of a tendency toward mutualism 
between the fungus and host. McCue (22) observed that tomato 
plants treated with phosphatic fertilizers developed less leaf 
blight (presumably Septoria) than control plants, while those on 
nitrogen and potash plots were more heavily infected than the 
controls. At the same time the highest yields (showing greatest 
vigor of growth), were obtained from the plants which received 
nitrogen and potash. Norton (24) also noted a decrease in infec- 
tion by Septoria on tomatoes treated with phosphates. 

Cereals grown by Spinks (37) in nutrient solution seemed to be 
susceptible to infection by Erysiphe in proportion as vigorous 
growth of the host was maintained. Excess of phosphates and 
potash diminished susceptibility while nitrates increased it. 
Stakman (38) noted that heavy manuring of rye increased the 
number of successful inoculations with Puccinia graminis Avenae- 
Even the seed fungus, however, is surpassed in some mutualistic 
characters by the mycorrhizas and lichens. The mycorrhizas 
show grades of interrelation between host and parasite from active 
parasitism to finely adjusted mutualism. Stout (39) has shown 
that Sclerotium rhizodes may be parasitic on aerial parts of Calama- 
grostis canadensis and at the same time assume a mycorrhizal 
habit on the roots of the same host. Nieuberg (23) found that in 
the lichens mutualism persists for a long period perhaps even 
after the fungus finally penetrates the algal cells. This habit is a 
close approach to that of producing haustoria as in the more 
specialized parasites. 

With the exception of the nitrogen fixing bacteria, only the 
mycorrhizal and lichen fungi have been proven to contribute 
anything of value to the host. It is important, however, in the 
development of methods of avoiding disease in plants, as well as 
for a clearer understanding of the nature of parasitism, that it be 
recognized that the relation of host and parasite is not of necessity 



26 Thomas: Infection of Apium graveonens 

antagonistic throughout but may on the contrary become special- 
ized in such a way that infection and the development of the 
typical symptoms of the disease are directly favored by the general 
vigor of the host plant. 

In the relation of the age of different tissue on the same celery 
plant to infection, it seems that there are two entirely separate 
conditions operating, one which governs the establishment of 
infection and another which determines the rate of subsequent 
spread of the mycelium. The first stage seems closely related to 
the immediate metabolic activity of the host cells especially in 
view of the marked increase in the number of infections produced 
by the addition of fertilizers to the plant at the time of inoculation. 
That starch metabolism has no very direct relation to infection 
is indicated by the results of inoculating etiolated plants. 

Pool and McKay (28) state that the infection of Beta vulgaris 
by Cercospora beticola is closely related to, if not directly controlled 
by the movement of the stomata. According to their data 
(pp. 1019, 1031), however, heart leaves which are said not to be 
infected show an average stomatal pore width of from o fx to 9 n 
between 10 a.m. and 1 p.m., only one case in ten falling below 
2.5 /x. Celery leaves can be infected as soon as they come into 
view but they frequently reach their mature height before the 
spots are visible. It is not clear whether the incubation period is 
taken into consideration in this connection by Pool and McKay. 
They do not show comparative counts on old and young leaves 
from a single infection. These authors show a close correlation 
between the maturity of the leaf and the number of stomata per 
unit area. Ensign (10a) has shown a very definite relation between 
the size of the vein-islets and maturity of the leaf in citrus. The 
relation of infection to age of the host as determined by these 
criteria deserves further attention. 

It has been pointed out that the tissue between spots separated 
by a given distance will break down more rapidly on old leaves 
than on young ones. It has been noted furthermore that from a 
single inoculation the spots are visible on the old leaves one to 
several days before they can be seen on the younger leaves. 

The Septoria produces a vigorous growth on widely different 
culture media, as well as on cooked celery, but at the same time 



Thomas: Infection of Apium graveolens 27 

is unable to develop on a living host as nearly related to celery as is 
parsley. 

The comparatively narrow specialization of the Septoria on 
celery suggests a promising outlook for experiments in breeding 
for resistance. More intensive work in this direction is needed. 

This work was done in the laboratory of Professor R. A. 

Harper and acknowledgment is herewith made to him for numerous 

helpful criticisms and suggestions. 
Department of Botany 
Columbia University 

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28 Thomas: Infection of Apium graveolens 

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HARVEY EARL THOMAS 

Born November 18, 1890, at Grant, Virginia; graduated Acad- 
emy of Idaho, 191 1; Bachelor of Science, Virginia Polytechnic In- 
stitute, 191 5; Master of Science, Virginia Polytechnic Institute, 191 6. 
Author of: Dusting for peach scab (Report Virginia Experiment 
Station, 191 5-16); Rust cultures of Aecidium tubulosum and A. 
passifloriicola (Phytopathology 8: 163-164); Report of the Plant 
Pathologist (Annual Report Porto Rico Agricultural Experiment 
Station, 191 7). 



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